Method and apparatus for providing positive airway pressure to a patient

ABSTRACT

A system including methods and apparatus for treatment of a medical disorder such as obstructive sleep apnea or congestive heart failure. The system involves applying a gain to flow rate of pressurized gas delivered to a patient during inspiratory and/or expiratory phases of a respiratory cycle to deliver the pressurized gas in proportion to the respective gains during inspiration and/or expiration. A base pressure may be applied in addition to the gain-modified pressures and an elevated pressure profile may be employed to assist or control inspiration. The system may be fully automated responsive to feedback provided by a flow sensor that determines the estimated patient flow rate. A leak computer can be included to instantaneously calculate gas leakage from the system. The system may be utilized in connection with conventional continuous positive airway pressure treatments, such as CPAP or bi-level positive airway pressure equipment to effect various beneficial treatment applications.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This is a continuation-in-part of U.S. patent application Ser.No. 09/610,733 filed Jul. 6, 2000, which is a continuation of U.S.patent application Ser. No. 09/041,195 filed Mar. 12, 1998, now U.S.Pat. No. 6,105,575, which is a continuation-in-part of U.S. patentapplication Ser. No. 08/679,898 filed Jul. 15, 1996, which is acontinuation-in-part of application Ser. No. 08/253,496 filed Jun. 3,1994, now U.S. Pat. No. 5,535,738.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to methods and apparatusfor treating breathing and/or cardiac disorders and, more particularly,to methods and apparatus for providing a pressure to an airway of apatient during at least a portion of the breathing cycle to treatobstructive sleep apnea syndrome, chronic obstructive pulmonary disease,congestive heart failure, and other respiratory and/or breathingdisorders.

[0004] 2. Description of the Related Art

[0005] During obstructive sleep apnea syndrome (OSAS), the airway isprone to narrowing and/or collapse while the patient sleeps. Continuouspositive airway pressure (CPAP) therapy seeks to avoid this narrowing bysupplying pressure to splint the airway open. With CPAP, this splintingpressure is constant and is optimized during a sleep study to besufficient in magnitude to prevent narrowing of the airway. Providing aconstant splinting pressure, i.e., CPAP, is a simple solution to theproblem posed by the collapsing airway. However, this approach exposesthe patient to pressures that are higher than the pressures needed tosupport the airway for most of the breathing cycle.

[0006] During inspiration, the pressure created within the lungs islower than the pressure at the nose. This pressure difference drives theflow of air into the lungs. This pressure difference creates a pressuregradient in the airway connecting the lungs with the nose. That is tosay, the nose is typically at ambient pressure while the lungs andairway of the patient are at sub-ambient or negative pressures. Thisnegative pressure acts upon the airway and contributes to its collapse.CPAP levels are typically set to raise the pressure level in the entirerespiratory system to the level required to both eliminate thesub-ambient pressures generated by inspiration and overcome anymechanical collapsing forces that result from the structure of theairway tissues, muscle tone, and body position. The inspiratorypressures, i.e., inspiratory positive airway pressure or “IPAP,” inbi-level positive airway pressure systems are set in a similar manner.

[0007] During exhalation, a positive pressure gradient exists betweenthe interior of the lungs and the exterior of the body. This positivepressure gradient helps to support the airway during exhalation. At theend of exhalation, the pressure gradient is essentially zero; flow islikewise zero and the airway is unaffected by respiratory efforts. Anycollapse of the airway at the end of exhalation is purely a function ofthe structure of the airway tissues, muscle tone, and body position.Bi-level devices seek to supply the expiratory pressure required tosupport the airway at the end of exhalation.

[0008] It should be noted that over the course of a breathing cycle, thepressure gradients between the lungs and the exterior of the body arenot constant. The inspiratory pressure gradient falls from zero at thestart of inspiration to a peak negative value and then rises back tozero at the end of inspiration. The expiratory pressure gradient risesfrom zero at the start of exhalation to a peak value and then falls backto zero as exhalation ends. Because the pressure gradient varies overthe breathing cycle, the pressure necessary to overcome airway collapseshould ideally vary over the breathing cycle.

[0009] Traditional CPAP therapy ignores these variations in pressurerequirements and provides therapy at one pressure level. ConventionalCPAP is rather crude and offers far from optimal therapy since the CPAPpressure is based solely on a worst-case treatment parameter, i.e., thepeak pressure requirements during inspiration.

[0010] Representing an advancement over conventional CPAP, bi-levelpositive airway pressure (bi-level PAP) therapies seek to take advantageof the different pressure requirements to lower the pressure duringexhalation. Nevertheless, bi-level therapies also fail to afford optimaltreatment because the inspiratory positive airway pressure (IPAP) ofbi-level PAP is again based on the patient's peak needs encounteredduring inspiration and remains constant over the entire inspiratoryphase respiration. Also, during bi-level treatment, the expiratoryposition airway pressure (EPAP) remains constant and is related solelyto the support needs at the end of exhalation.

[0011] In addition to OSAS, positive airway pressure therapy, such asbi-level PAP therapy, has been applied in the treatment of otherbreathing disorders, such as chronic obstructive pulmonary disorder(COPD). One of the problems with this mode of treatment, however, isthat the patient has difficulty stopping inspiratory flow. Thisphenomenon arises due to the disparity between applied IPAP and thepressure needed to overcome the patient's respiratory resistance at theend of inspiration. As the former pressure typically exceeds the latter,the “surplus” IPAP at the end of inspiration leads to uncomfortable andpotentially harmful hyperinflation of the patient lungs.

[0012] Conversely, in order to begin inspiratory flow, a COPD patientmust reduce the pressure inside his lungs to a pressure that is lessthan the ambient pressure at the inlet of his respiratory system. Due tothe condition commonly known as “Auto-PEEP,” the pressure in thepatient's lungs is typically above ambient pressure at the end ofexhalation. The patient's breathing muscles thus must perform additionalwork to expand the lungs and thereby reduce lung pressure below ambientbefore flow into the lungs can occur. Auto-PEEP is typically treatedwith a form of resistive counter pressure known as PEEP (positive endexpiratory pressure). PEEP is set at a level just below the patient'sAuto-PEEP level, thereby reducing the amount of breathing work requiredto initiate inspiratory flow.

[0013] With conventional treatments, such as pressure support, CPAP orbi-level therapy, PEEP is achieved by applying the same pressure overthe entire phase of expiration, e.g., the EPAP phase of bi-level PAPtherapy. It should be noted that EPAP is not synonymous with PEEP. EPAPindicates a constant pressure delivered to the patient throughoutexhalation, while PEEP indicates positive end expiratory pressure. Bydefinition, the PEEP pressure is only required at the end of exhalation.As such, the administration of EPAP throughout the expiratory cycle toassure that satisfactory PEEP is maintained undesirably contributes tothe breathing work that a patient must perform during exhalation.

[0014] In addition to CPAP and bi-level PAP, other systems have beenproposed for clinical research and/or therapeutic application, includingtreatment of OSAS, COPD and other breathing disorders, that offer anassortment of methods and apparatus by means of which a subject'srespiratory efforts may be induced, superseded, assisted and/orresisted. Some of these systems perform their prescribed functionsresponsive to one or more parameters associated with a subject'srespiratory activity including, but not limited to, inspiratory and/orexpiratory flow, inspiratory and/or expiratory pressure, tidal volumeand symptoms indicative of airway obstruction, e.g., snoring sounds.Some achieve their objectives transthoracically while others deliver airat positive or negative pressure directly to the subject's airway.

[0015] An early example of such a system, commonly referred to as an“iron lung,” is disclosed in a publication entitled “MechanicalAssistance to Respiration in Emphysema, Results with aPatient-Controlled Servorespirator,” authored by James R. Harries, M.D.and John M. Tyler, M.D., published in the American Journal of Medicine,Vol. 36, pp. 68-78, January 1964. The iron lung proposed in thatpublication is a respirator designed to apply and remove transthoracicpressure to and from the exterior surface of the body of a subject whosits in a large pressurizable chamber in order to assist the patient'srespiratory efforts (i.e., the iron lung applies negative pressureduring inspiration and either ambient or positive pressure duringexpiration). Sophisticated for its day, the apparatus continuallycontrolled the internal chamber pressure in response to the patient'sspontaneous respiration, specifically in response to detectedrespiratory flow or volume. Indeed, a signal obtained from a straingauge pneumograph fastened around the patient's chest was electricallyseparated into three components: one proportional to volume, another toinspiratory flow and a third to expiratory flow. Each component wasassigned a separate gain control. The component signals are thenrecombined to control the pressure in the chamber by means of anelectrically driven variable valve situated between a blower and thechamber.

[0016] Although effective for their intended purposes, this and otheriron lung devices have generally fallen into disfavor because of theirbulk, inconvenience, cost and limited application. That is to say,because of their size and cost such equipment is purchased andmaintained essentially exclusively by medical facilities such ashospitals and clinics. Further, iron lungs do not lend themselves totreatment of OSAS and related disorders where comfort andunobtrusiveness are critical for patient compliance and treatmentefficacy. This is because negative pressure applied during inspirationcompounds the factors that operate to collapse the airway during aninspiratory phase.

[0017] An essay entitled, “An Apparatus for Altering the Mechanical Loadof the Respiratory System,” authored by M. Younes, D. Bilan, D. Jung andH. Krokes, and published in 1987 by the American Physiological Society,pp. 2491-2499, discloses a system for loading and unloading of asubject's respiratory efforts to effect various respiratory responses.The system may load or unload during inspiration, expiration, or both,to assist or resist a subject's spontaneous respiratory activity. Thesystem may apply a continuous positive or negative pressure directly tothe subject's airway and loading or unloading occurs via a commandsignal generated by detected respiratory flow, volume, applied voltage,an external function, or other source.

[0018] A drawback to this system, however, is that a single resistivegain is chosen for resistive loading or unloading. This single gain isapplied to a “half-wave” of the respiratory cycle (either inspiration orexpiration) or the “full-wave” thereof (both inspiration andexpiration). In other words, under full-wave respiratory loading orunloading, a single chosen gain value is employed during bothinspiration and expiration. Thus, a gain that may produce favorableresults in regard to reducing breathing work during inspiration, forexample, may cause less than desirable or even detrimental consequencesduring expiration. The converse is true for a gain selected specificallyfor optimizing expiratory work reduction.

[0019] In addition, the Younes et al. system operates as a closed,leak-proof system. Hence, to predict its ability to function in an open,leak-tolerant system would be problematic. As such, whether it may beadapted to OSAS treatment, which invariably involves some degree ofknown and unavoidable unknown system leakage, is suspect.

[0020] U.S. Pat. No. 5,107,830 to Younes essentially reiterates all ofthe “breathing assist” (unloading) disclosure that is covered in theYounes, et al. American Physiological Society publication discussedabove. In the system disclosed in U.S. Pat. No. 5,107,830, however, theadjustable pressure gain is only realized during inspiration becausepressure output is set to zero during exhalation. Additionally, outputpressure Is calculated as a function of both detected patientinspiratory flow and volume. Furthermore, the system is applicable toCOPD but not OSAS therapy.

[0021] An article entitled “A Device to Provide Respiratory-MechanicalUnloading,” authored by Chi-sang Poon and Susan A. Ward and published inMarch 1987 in IEEE Transactions on Biomedical Engineering, Vol. BME-33,No. 3, pp. 361-365, is directed to an apparatus which functions somewhatsimilar to one mode of operation described in both Younes disclosures.That is, the Poon, et al. device may operate to unload a subject'sbreathing, but only during inspiration. Poon, et al. provide theirinspiratory assistance by establishing a positive mouth pressurethroughout inspiration in a constant proportion to instantaneous flow.The constant proportion is achieved by (1) selecting a desired gain fora detected positive mouth pressure signal, (2) calculating the ratio ofthe gain-modified mouth pressure signal over a detected signalreflecting instantaneous flow, (3) comparing the calculated ratio to aselected reference ratio to generate a valve motor control signal, and(4) using the valve motor control signal to operate a motor that drivesa servo valve to control the positive pressure applied to the subject'sairway. Thus, the apparatus output pressure is determined as a functionof both detected pressure and flow. Further, the pressure must be outputat a value sufficient to maintain a constant ratio of pressure to flow.

[0022] A publication entitled “Servo Respirator Constructed from aPositive-Pressure Ventilator,” by John E. Remmers and Henry Gautier,which was published in August, 1976 in the Journal of AppliedPhysiology, Vol. 41, No. 2, pp. 252-255, describes a modified ventilatorthat may function as a “demand” respirator generating a transthoracicpressure proportional to phrenic efferent respiratory discharge. Phrenicefferent respiratory discharge is an indication of the outgoing brainsignal to the phrenic nerve, which controls diaphragm function. Aphrenic efferent respiratory discharge signal causes the diaphragm tocontract whereby the subject exerts an inspiratory effort. The phrenicefferent respiratory discharge serves as the apparatus command signaland is processed to produce a moving time average (MTA) and thesubject's tracheal pressure serves as a negative feedback signal. Likethe Poon et al. device, the Remmers et al. apparatus providesrespiratory assistance only during inspiration.

[0023] An apparatus for automatically regulating the flow and pressureoutput of a respirator is disclosed in U.S. Pat. No. 3,961,627 to Ernstet al. Like the aforementioned Poon et al. device, however, the Ernst etal. apparatus relies upon an unduly complicated scheme dependent upondetected respiratory pressure and flow in calculating delivered outputflow and pressure. More particularly, Ernst et al. propose regulatingthe delivered flow and pressure of a respiration gas in a respiratorduring the respiration cycle in which the actual flow and pressure ofthe respiration gas are measured via a measuring device arrangedproximate a patient interface. The measured values are converted intoelectrical signals and the flow and pressure of the respiration gas arecontrolled during the inspiration and expiration portions of therespiration cycle via a valve arranged between a respiration gas sourceand the measuring device. The method for regulating the flow andpressure output comprises (1) measuring the actual flow of respirationgas proximate the patient, (2) measuring the actual pressure ofrespiration gas proximate the patient, (3) calculating nominal values offlow and pressure from preselected fixed values and the actual values,(4) comparing the actual values measured for the flow and pressure withthe nominal values, and (5) obtaining from the comparison a controlsignal for modulating the valve and thereby regulating the flow andpressure of the respiration gas.

[0024] Additionally, apart from its utilization of two detectedrespiratory parameters (flow and pressure) and the complex manner inwhich these and other variables are reiteratively processed to produceapparatus flow and pressure output, the Ernst et al. system, althoughcapable of delivering a base pressure equivalent to a patient's requiredend expiratory pressure, is nevertheless unable to deliver any pressureless than the base pressure. Consequently, the Ernst et al. apparatusrequires the patient to perform more breathing work than is necessary tosatisfy his respiratory needs, especially in the expiratory phase of arespiration cycle, thereby deleteriously affecting the patient's comfortand likelihood of continued compliance with the treatment.

[0025] In addition to the treatment of breathing disorders, positiveairway pressure therapy has been applied to the treatment of congestiveheart failure (CHF). In using CPAP on CHF, the effect of the CPAP is toraise the pressure in the chest cavity surrounding the heart. This hasthe impact of reducing the amount of pressure the heart has to pumpagainst to move blood into the body. By reducing the pressure the heartworks against, the work required of the heart is reduced. This allowsthe sick heart to rest and potentially to get better.

[0026] The pressure in the chest cavity is also impacted by respirationeffort. With inspiration, the pressure in the chest is reduced (negativerelative to resting pressure) due to inspiratory effort. This forces theheart to pump harder to move blood into the body. With expiration, thepressure in the chest is slightly increased (positive relative toresting pressure) due to the elastic properties of the chest. Thisallows the heart to decrease its efforts to pump blood. Whileconventional CPAP can help the heart rest, it has negative aspects forthe patient such as increased work of exhalation and discomfort from thepressure.

SUMMARY OF THE INVENTION

[0027] It is an object of the present invention to provide anuncomplicated system operable to deliver pressurized air to the airwayof a patient and readily adaptable to the treatment of OSAS, COPD andother respiratory and/or pulmonary disorders that does not suffer fromthe disadvantages of conventional pressure application techniques. Thisobject is achieved by providing an apparatus for delivering pressurizedbreathing gas to an airway of a patient. The apparatus, which isreferred to below as a “proportional positive airway pressure” or “PPAP”apparatus, includes a gas flow generator, a patient interface thatcouples the gas flow generator to the patient's airway, a sensor thatdetects a fluid characteristic associated with a flow of gas within thepatient interface, a pressure controller that regulates the pressure ofbreathing gas provided to the patient, and a control unit that controlsthe pressure controller.

[0028] The control unit controls the pressure controller so that thebreathing gas is delivered to the patient at a minimally sufficientpressure during at least a portion of a breathing cycle to preventairway collapse, in which case the minimally sufficient pressure is asummation of a pressure needed to prevent airway collapse due tomechanical forces resulting from the structures of the patient and apressure needed to overcome airway collapse due to respiratory effort.The apparatus also includes a selector unit that establishes a firstgain. The control unit controls the pressure controller so as to deliverthe breathing gas at the minimally sufficient pressure during at least aportion of the breathing cycle based on the first gain and the signalfrom the sensor.

[0029] The PPAP system of the present invention provides airway pressurethat is lower than pressures typically necessary to treat OSAS, which isnormally treated using conventional CPAP or bi-level PAP therapy. WithPPAP, the patient receives exhalation pressures lower than conventionalbi-level PAP expiratory positive airway pressure levels and well belowconventional CPAP levels. Also, the average pressure delivered duringinspiration can be lower than conventional or bi-level PAP inspiratorypositive airway pressure or CPAP levels, whereas peak PPAP pressure isroughly equivalent to conventional IPAP or CPAP levels. The PPAPpressure range (peak inspiratory pressure to minimum expiratorypressure) is generally between 2 to 20 cm H₂O, with typical values inthe 8 to 14 cm H₂O range. This is consistent with bi-level PAP therapywhere significant comfort/compliance is found with peak inspiratory tominimum expiratory pressure differentials of 6 cm H₂O or more. Thecomplexity of titration using the apparatus of the instant invention isroughly equivalent to current bi-level PAP titration. In addition, thetitration system may incorporate a feedback circuit to provide fullyautomated PPAP.

[0030] Similar to treatment of OSAS, PPAP also delivers mean airwaypressure that is lower than pressures typically necessary to treat COPDusing conventional bi-level PAP therapy with PEEP or proportional assistventilation (PAV) with PEEP. That is, with PPAP, the patient receivesaverage exhalation pressures lower than conventional EPAP levels,average inspiration pressures lower than conventional IPAP, and peakPPAP pressure roughly equivalent to conventional IPAP pressures andconventional peak PAV levels. Hence, less breathing work is requiredwith PPAP than with conventional PAV or bi-level treatments of COPD orOSAS.

[0031] It is a further object of the present invention to provide amodified CPAP apparatus that is capable of easily detecting exhalationand modifying the exhalation pressure to match a selected pressureprofile. This object is achieved by providing an apparatus that includesa gas flow generator, a patient interface that couples the gas flowgenerator to the patient's airway, a sensor that detects a physiologicalcondition that is suitable for use to differentiate between anexpiratory phase and an inspiratory phase of a breathing cycle, apressure controller that regulates the pressure of breathing gasprovided to the patient, and a control unit that controls the pressurecontroller. More specifically, the control unit causes the breathing gasto be delivered at a first pressure level during an inspiratory phase ofthe breathing cycle, which is consistent with the operation of aconventional CPAP device. However, the control unit causes the breathinggas to be delivered in accordance with a predetermined pressure profileduring the expiratory phase of the breathing cycle. This profileprovides a decrease in the EPAP provided to the patient. Because thepressure profile can be obtained by controlling the operation ofexisting CPAP devices, it can be readily implemented on many suchdevices, thereby providing a better therapy for a patient using existingdevices.

[0032] It is yet another object of the present invention to provide asystem for eliminating oscillations in the flow provided during patientexhalation that can occur with use of the PPAP device. According to afirst embodiment of the present invention, this object is achieved bycausing the pressure controller to provide a pressure to the patientduring expiration that is the greater of (1) a first minimallysufficient pressure that is determined by applying a gain to the signaloutput by the sensor and (2) a second minimally sufficient pressure thatcorresponds to a current pressure being provided to the patient. Byensuring that the pressure provided to the patient is always the greaterof these two pressures, the pressure received by the patient duringexpiration does not oscillate, because should the pressure to beprovided to the patient begin to decrease below the current pressure,the device will not use the calculated pressure, but will continue toprovide the patient with the current pressure, thereby preventing apressure decrease below the current pressure.

[0033] According to a second embodiment of the present invention, theobject of preventing oscillations in the patient flow provided duringexpiration is achieved by causing the pressure controller to provide anexpiration pressure that is determined based on a volume of gas to beexhaled and a gain. This gain can be the same gain or a different gainfrom that applied to the signal from the sensor during inspiration (ifany). The volume of gas to be exhaled corresponds to a differencebetween the current volume of gas in the patient and the volume of gasin the patient at rest.

[0034] These and other objects, features and characteristics of thepresent invention, as well as the methods of operation and functions ofthe related elements of structure and the combination of parts andeconomies of manufacture, will become more apparent upon considerationof the following description and the appended claims with reference tothe accompanying drawings, all of which form a part of thisspecification, wherein like reference numerals designate correspondingparts in the various figures. It is to be expressly understood, however,that the drawings are for the purpose of illustration and descriptiononly and are not intended as a definition of the limits of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035]FIG. 1 is a functional block diagram of an apparatus according tothe instant invention;

[0036]FIG. 2 is a functional block diagram of a further embodiment of anapparatus according to the instant invention;

[0037]FIGS. 3A and 3B are flow and pressure diagrams, respectively,graphically representing the general manner in which an apparatusaccording to the instant invention outputs pressurized breathing gas ina proportional relation to the patient flow in both the inspiratory andexpiratory phases of a single respiratory cycle;

[0038]FIGS. 4A and 4B are flow and pressure diagrams, respectively,similar to FIGS. 3A and 3B, exemplifying a number of apparatus outputpressure curves that are achieved through selective adjustment ofinspiratory and expiratory gain setting controls of the proportionalpositive airway pressure circuitry of the instant invention;

[0039]FIGS. 5A and 5B are flow and pressure diagrams, respectively,similar to FIGS. 3A and 3B, contrasting a pressure output curve typicalof an apparatus according to the instant invention with pressure outputcurves of a conventional respiratory assistance apparatus;

[0040]FIGS. 6A and 6B are flow and pressure diagrams, respectively,similar to FIGS. 3A and 3B, depicting alternative pressure profiles thatare employed at the beginning of an inspiratory phase of respiration tofacilitate the onset of inspiration;

[0041]FIGS. 7A and 7B are flow and pressure diagrams, respectively,similar to FIGS. 3A and 3B, illustrating a resultant apparatus pressureoutput curve according to a further embodiment of the present invention;

[0042]FIGS. 8A and 8B are flow and pressure diagrams, respectively,similar to FIGS. 3A and 3B, showing a resultant apparatus pressureoutput curve achieved by combing a conventional bi-level positive airwaypressure therapy with the proportional positive airway pressure therapyaccording to the instant invention;

[0043]FIGS. 9A and 9B are flow and pressure diagrams, respectively,similar to FIGS. 3A and 3B, reflecting a further resultant apparatuspressure output curve achieved by combining a conventional bi-levelpositive airway pressure therapy with proportional positive airwaypressure therapy according to the instant invention;

[0044]FIG. 10 is a functional block diagram of a further embodiment ofan apparatus according to the instant invention;

[0045]FIGS. 11A and 11B are flow and pressure diagrams, respectively,similar to FIGS. 7A and 7B, illustrating a resultant apparatus pressureoutput curve according to a further embodiment of the present inventionthat utilizes a simplified pressure profile generating technique;

[0046]FIG. 12 is a pressure diagram illustrating the occurrence ofoscillations in the pressure provided to the patient during exhalation;

[0047]FIG. 13 is a pressure diagram illustrating a first technique forreducing the oscillations illustrated in FIG. 12;

[0048]FIG. 14 is a pressure diagram illustrating a second technique forreducing the oscillations illustrated in FIG. 12; and

[0049]FIG. 15 is a pressure and flow diagram illustrating a furthertechnique for handling pressure oscillations during the expiratory phaseof the breathing cycle.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

[0050] There is generally indicated at 10 in FIG. 1 a proportionalpositive airway pressure apparatus according to a presently preferredembodiment of the instant invention and shown in the form of afunctional block diagram. Apparatus 10 is operable according to a novelprocess to deliver breathing gas, such as air, oxygen or a mixturethereof, at relatively higher and lower pressures (i.e., generally equalto or above ambient atmospheric pressure) to a patient 12 in proportionto the patient's respiratory flow for treatment of OSAS, snoring, CHF,COPD and other respiratory or cardio-respiratory disorders.

[0051] Apparatus 10 includes a gas flow generator 14, such as aconventional CPAP or bi-level PAP blower, i.e., a centrifugal blower,that receives breathing gas from any suitable source, e.g., apressurized bottle 16 of oxygen or air, the ambient atmosphere, or acombination thereof. The gas flow from flow generator 14 is passed via adelivery conduit 18 to a breathing appliance or patient interface 20 ofany suitable known construction that is worn by patient 12. In anexemplary embodiment of the present invention, the conduit 18 is a largebore flexible tube and the patient interface 20 is either a nasal maskor a full face mask, as shown. Other breathing appliances that may beused in lieu of a mask include a mouthpiece, a nasal seal, nasal prongsor cannulae, an endotracheal tube, a trachea adapter or any othersuitable appliance for interfacing between a source of breathing gas anda patient. Also, the phrase “patient interface” can encompass more thatthe interface worn by the patient. For example, the patient interfacecan include delivery conduit 18 and any other structures that connectthe source of pressurized breathing gas to the patient.

[0052] The apparatus also includes a sensor, such as a flow transducer22 or similar flow sensing element, situated within or near thebreathing circuit, i.e., the patient interface 20, conduit 18 or gasflow generator 14. Flow transducer 22 may be any suitable gas flowmeter, such as, for example, a bidirectional dynamic mass flow sensor.Preferably, however, the flow transducer is a pressure responsive sensorfor detecting the magnitude of the pressure gradients between the inletof the patient's airway and his lungs. Within the scope of the presentinvention, flow and respiratory pressure gradient are highly correlated.

[0053] It is to be understood that the flow sensor need not be coupleddirectly to conduit 18. On the contrary, the present inventioncontemplates the use of any sensor or a plurality of sensors that canmeasure the flow of breathing gas in the conduit. For example, flow inthe system can be measured at the patient interface device or can bemeasured or estimated from the motor or piston speed, power, current,torque, or feedback control used to provide the elevated pressure byflow generator 14. Thus, the sensor need not be a direct flowmeasurement, but can be determined indirectly from measurementsassociated with the flow generator. In short, the present inventioncontemplates any conventional technique for measuring the flow of gasdelivered to the patient as flow transducer 22.

[0054] In accordance with a presently preferred embodiment, the flowtransducer 22 is interposed in line with conduit means 18, mostpreferably downstream of a pressure controller 24. The flow transducergenerates output signals that are provided, as indicated by referencenumeral 26, to PPAP circuitry 28 described in greater detailhereinafter. The output signals include first flow rate signalsindicative of inspiration by the patient and second flow rate signalsindicative of the patient's expiration. The signals are continuouslytransmitted and correspond to the instantaneous flow rate of breathinggas within conduit means 18.

[0055] In addition, the output from flow transducer 22 is also desirablyprovided, as indicated by reference numeral 30, to an optional leakdetecting system 32. A suitable leak detector for present purposes isthat disclosed in U.S. Pat. No. 5,148,802, the disclosure of which isincorporated herein by reference. However, other techniques forsubstantially instantaneously calculating system leakage, including bothknown leakage, such as that discharged through a mask exhaust port 34,and unknown leakage, such as that at various conduit couplings or at thepatient contact site of the patient interface 20, are acceptable. Withany non-invasive embodiment of the present invention, i.e., notinvolving an endotracheal tube or trachea adapter, the patient flow mustbe estimated taking into account the aforesaid known and unknown systemleaks.

[0056] The output signal from the leak detecting system 32 is provide,as at 36, to PPAP circuitry 28. In this way, the PPAP circuitry logiccontinuously compares the output from flow transducer 22 with that fromleak detecting system 32 to discriminate that portion of system flowassociated with the patient's respiration from that caused by systemleakage. As a result, PPAP circuitry 28 more precisely controls theoutput of the pressure controller 24 as a function of patientrespiratory flow, rather than overall system flow.

[0057] If formed as a mask, as illustrated, patient interface 20commonly includes, as mentioned above, a suitable exhaust system,schematically indicated at 34, to exhaust breathing gases duringexpiration. Exhaust system 34 preferably is a continuously open portthat imposes a suitable flow resistance upon exhaust gas flow to permitpressure controller 24, located in line with conduit 18 between flowgenerator 14 and patient interface 20, to control the pressure of airflow within the conduit and thus within the airway of the patient. Forexample, exhaust port 34 may be of sufficient cross-sectional flow areato sustain a continuous exhaust flow of approximately 15 liters perminute at a system pressure of 10 cm H₂O. The flow via exhaust port 34is one component, and, typically, the major component of the overallsystem leakage, which is an important parameter of system operation. Inan alternative embodiment, it has been found that a non-rebreathingvalve may be substituted for the continuously open port.

[0058] Pressure controller 24 controls the pressure of breathing gaswithin conduit 18 and thus within the airway of the patient. Pressurecontroller 24 is located preferably, although not necessarily,downstream of flow generator 14 and may take the form of an adjustable,electronically-controlled valve. Of course, other method of pressurecontrol are contemplated by the present invention, such as controllingthe operation of the pressure generator, either alone or in combinationwith a pressure control valve. For example, one embodiment of thepresent invention contemplates that the pressure generator is a blower,which includes an impeller mounted in a housing and rotated by a motor.In this embodiment, the pressure controller is a control system thatcontrols the operating speed of the motor in the blower without the needfor a dedicated flow control valve.

[0059] Apparatus 10 also desirably includes a safety circuit, preferablycomprising an adjustable maximum pressure setting control 38 and anadjustable minimum pressure setting control 40 operatively connected topressure controller 24. The safety circuit allows the manufacturer, thepatient or his overseeing health care professional to selectivelyestablish minimum and maximum system output pressures below and abovewhich the system will not dispense pressurized gas. The minimum pressurewill, of course, be at least zero and, preferably, a threshold pressuresufficient to maintain pharyngeal patency during expiration. The maximumpressure, on the other hand, will be a pressure somewhat less than thatwhich would result in over-inflation and perhaps rupture of thepatient's lungs. The safety circuit functions differently than thepressure controls which determine, for instance, the CPAP prescriptionpressure or the IPAP and EPAP prescription pressures used in bi-levelPAP therapy. That is, instead of establishing lower and upperprescription pressures to be administered during normal usage of theapparatus (subject to the influence of the PPAP circuitry 28), themaximum and minimum pressure setting controls 38 and 40 set absoluteminimum and maximum fail-safe output pressure limits which are not to beexceeded. Thus, the danger of potential physical harm to the patient inthe even of malfunction of other system components, e.g., theprescription pressure controls, is effectively eliminated.

[0060] PPAP circuitry 28, according to the present invention, is subjectto the influence of additional essential controls, including a basepressure control 42, an inspiratory gain setting control 44, and anexpiratory gain setting control 46. The base pressure control 42establishes a base pressure (Pbase), usually greater than or equal tozero and conceptually equal to the EPAP level in bi-level therapy,sufficient to maintain airway patency at the beginning and end ofexhalation. The inspiratory gain setting control 44 permits selection ofa resistive gain (Gain_(Insp)) to be applied to the detected inspiratoryflow. Similarly, the expiratory gain setting control 46 enablesselection of a resistive gain (Gain_(Insp)) to be applied to thedetected expiratory flow.

[0061] In a broad sense, PPAP therapy and the PPAP apparatus 10constitute a novel system providing pressure to a patient via nasal,nasal/oral, oral, or trachea interface to treat OSAS, COPD and otherbreathing disorders. The pressure delivered to the patient is a functionof the patient flow rate. The function can be described as follows:

Pdelivered=Pbase+Gain*Flow

[0062] where:

[0063] “Pdelivered” is the pressure delivered to the patient interface;

[0064] “Pbase” is the base line pressure (greater than or equal to zeroand conceptually equal to EPAP);

[0065] “Flow” is the estimated patient flow rate determined by the flowtransducer; and

[0066] “Gain” is the constant used to augment pressure based on the flowrate. The gain constant can further be refined to allow one constant forinspiration (positive flow) and a different constant for exhalation(negative flow).

[0067]FIGS. 3A and 3B represent flow and pressure diagrams,respectively, graphically depicting the manner in which apparatus 10outputs pressurized breathing gas in proportional relation to patientflow, as detected by flow transducer 22, in both the inspiratory andexpiratory phases of a respiratory cycle. The pressure curve of FIG. 3Breflects a situation where the same gain is chosen for both inspiratoryand expiratory flow. Conceivably, essentially the same pressure curvemay be generated by the apparatus disclosed in the aforementioned essayentitled “An Apparatus for Altering the Mechanical Load of theRespiratory System,” by Younes, et al. which may use a single resistivegain applicable to both inspiration and expiration.

[0068] With PPAP apparatus 10, however, separate and independent gainsmay be chosen for inspiration and expiration, whereby gains best suitedto optimizing performance, i.e., minimizing breathing work, may beprecisely matched with each of the inspiratory and expiratory phases.Thus, the function of the apparatus described in the Younes et al.article corresponds to a special and relatively limited application ofthe present invention where the selected inspiratory and expiratorygains are identical.

[0069] As is far more often the case, however, an optimum inspiratorygain is not the optimum expiratory gain and vice versa. Thus, thepressure output of the PPAP apparatus 10 is more accurately describedaccording to the following functions, which functions can be encodedinto the PPAP circuitry 28.

Pinhalation=Pbase+Gain_(Insp)*Flow,

and

Pexhalation=Pbase+Gain_(Exp)*Flow,

[0070] where:

[0071] “Gain_(Insp)” is the constant used during inspiration (positiveflow) to boost pressure based on the flow rate; and

[0072] “Gain_(Exp)” is the constant used during exhalation (negativeflow) to reduce pressure based on the flow rate.

[0073] The gain typically selected has a range of about 0 to 10 cmH₂O/liter/second for inspiration. The gain chosen for exhalation isnormally lower than the inspiratory gain, e.g., values in the range of 0to 4 cm H₂O/liter/second, although higher gain values may be chosen forinspiration and/or expiration, if such is desired or necessary.

[0074] Regardless of the chosen gain values, applying a flow signalderived from a normal respiratory pattern will result in a pressure riseabove Pbase during inspiration and will drop below Pbase duringexhalation. When patient flow is near zero, i.e., at the beginning andend of inspiration, as well as the beginning and end of exhalation, theoutput pressure approaches Pbase.

[0075]FIGS. 4A and 4B perhaps most clearly exemplify the effect that theselection of different gains for both the inspiratory and expiratoryphases of a respiratory cycle has upon the pressure output curve.Gain_(Insp(a)), Gain_(Insp(b)), Gain_(Insp(c)) and Gain_(Insp(d))represent, in descending order, several of an infinite range of gainvalues that may be applied during inspiration. Similarly, Gain_(Exp(e)),Gain_(Exp(f)), Gain_(Exp(g)) and Gain_(Exp(h)) indicate increasingexpiratory gain values. With different gain settings, any number of waveforms can be generated. For example, a high setting may be establishedfor Gain_(Insp) and a low setting for Gain_(Exp), or vice versa, or thegain settings for inspiratory flow and expiratory flow may be the same.

[0076] In one embodiment of the present invention, PPAP therapy seeks toprovide only the pressure that is necessary to prevent airway collapseat any given moment during the breathing cycle. This will generallyresult in supplying, at appropriate times, maximum pressure only whenpeak negative airway inspiratory pressures are detected and minimumpressure only when peak positive airway exhalation pressures aredetected. At all other times during the breathing cycle, the PPAPapparatus delivers air at a variable pressure responsive to thepatient's respiratory efforts in a range between the maximum and minimumpressures. As mentioned above, PPAP therapy also involves theadministration of a base pressure of zero or greater to which theproduct of a selected gain times instantaneous flow (inspiratory andexpiratory) is continuously added to produce the instantaneous outputpressure of the PPAP apparatus. An identical gain may be selected forinspiration and expiration, or different gain values may beindependently selected for inspiration and expiration. The base pressurewill be the pressure necessary to overcome any mechanical collapsingforces that result from the structure of the airway tissues, muscletone, and body position. In other words, the base pressure is generallyequivalent to the expiratory positive airway pressure or “EPAP”typically used in bi-level PPAP therapy.

[0077] In this connection, FIG. 5B illustrates the pressure output curvegenerated by the PPAP apparatus 10 vis-a-vis conventional CPAP andbi-level PAP apparatus over a single respiratory cycle. So long as theappropriate inspiratory and expiratory splint pressures are applied atpoint 48 (peak inspiratory flow), point 50 (beginning of exhalation) andpoint 52 (end of exhalation), less pressure may be provided at all othertimes during the breathing cycle than is normally supplied byconventional CPAP or bi-level PAP therapy. This reduced output pressureis represented by the “PPAP” curve of FIG. 5B. The hatched areas of thatfigure reflect the difference in pressures provided by PPAP and the IPAPand EPAP phases of bi-level PAP during a typical respiratory cycle. Thehatched areas may be conceptualized as the respiratory work or effortsavings that are attributed to PPAP. This work savings, as would beexpected, translates to greater comfort for the PPAP assisted patientand increased compliance with the respiratory treatment. According tothe present invention, PPAP therapy thus represents a novel respiratorydisorder treatment by which patient comfort (and, therefore, treatmentcompliance) exceed that offered by either CPAP or bi-level PAP therapy.

[0078] Referring again to FIG. 1, it will thus be appreciated thatpressure controller 24 is continuously governed by and outputs variablepressure responsive to a command signals 54 from PPAP circuitry 28.Command signals 54, in turn, are the product of the influences of one ormore of the outputs from flow transducer 22, leak detection system 32,base pressure control 42, inspiratory gain setting control 44,expiratory gain setting control and, in an alternative embodiment, apressure profile control 56 discussed below.

[0079] In normal breathing, a negative pressure gradient must begenerated before flow can begin. Hence, the negative pressure waveformgenerated in the airway must precede and thereby induce inspiratory flowat the start of inspiration. In an unstable airway, which ischaracteristic of OSAS, for example, this asynchronous relationship ofnegative pressure gradient and inspiratory flow onset would, if notaccommodated by suitable compensatory measures, lead to a situationwhere the PPAP therapy would not generate sufficient pressure (due tolow flow) to overcome the negative pressure in the airway, whereby totalor partial airway collapse may result. This problem can be solved by anumber of methods. For instance, a higher PPAP base pressure can be usedto provide additional pressure to support the airway at the beginning ofinspiration. Alternatively, however, as demonstrated by FIGS. 6A and 6B,a temporary pressure increase can be added at the start of inspirationto support to the airway until sufficient flow is generated to drive thePPAP process. The present invention offers several viable approaches bymeans of which pressure can be added during the initial phase ofinspiration to support the airway as inspiratory flow increases.

[0080] Temporary pressure increases may be effected using pressureprofile control 56 in operative connection with PPAP circuitry 28 toselect a desired elevated pressure waveform in the early stages ofinspiration. In this regard, pressure profiles may be used as minimumvalues for the output pressure at the outset of inspiration, therebygiving rise to the following alternative equations for available outputpressure during inspiration.

Pinhalation=greater of: Pbase+Gain_(Insp)*Flow or Pbase+Pprofile

[0081] where:

[0082] “Pinhalation” is the pressure delivered to the patient interfaceduring inspiration “Pbase” is the base line pressure (conceptually equalto EPAP);

[0083] “Flow” is the estimated patient flow;

[0084] “Gain_(Insp)” is the constant used during inspiration (positiveflow) to boost pressure based on the flow rate; and

[0085] “Pprofile” is a function that generates a pressure profile tosupport the airway at the start of inspiration. Such pressure profilefunctions may be constant, e.g., a step profile as shown by the dottedline identified by numeral 58 in FIG. 6B, time based (for instance, abackwards ramp profile as shown by the dotted and dashed line identifiedby numeral 60 in FIG. 6B), or any other functional shape.

[0086] Alternatively, pressure profiles can be used exclusively tocontrol the output pressure for a predetermined initial segment ofinspiration. The following equations represent system output pressureduring inspiration under such control conditions.

Pinhalation=Pprofile from start of breath to X

and

Pinhalation=Pbase+Gain_(Insp)*Flow from X to start of exhalation

[0087] where:

[0088] “Pinhalation” is the pressure delivered to the patient interfaceduring inspiration;

[0089] “Pbase” is the base line pressure (conceptually equal to EPAP);

[0090] “Flow” is the estimated patient flow;

[0091] “Gain_(Insp)” is the constant used during inspiration (positiveflow) to boost pressure based on the flow rate; and

[0092] “Pprofile” is any function that generates a pressure profile tosupport the airway at the start of inspiration. Such functions could beconstant, such as, for example, a step profile, or time based, such as abackwards ramp profile, or any other functional shape.

[0093] “X” is a preselected transition point determined by time, oranalysis of the flow signal, such as curvature, percent drop from peakflow rate, integration, derivative, analysis of prior breaths or acombination of flow analysis and time.

[0094] The PPAP apparatus 10 also has the capacity to measure and storein a memory 62 (FIG. 1) the following parameters: tidal volume,inspiratory time, expiratory time, peak pressure, peak flow, O₂saturation (as a voltage input from an external source), plural pressure(as a voltage input from an outside source), mask pressure, estimatedleakage, and system parameters, e.g., Pbase, Auto Gain_(Insp),Gain_(Insp), Gain_(Esp), IPAP and EPAP. It is to be understood that thislist is not exclusive; other parameters can be stored in memory 62.

[0095] A further method by which the present system addresses theproblem presented by the changing needs of the patient is to combine thebeneficial features of PPAP with a more controlled therapy such as CPAP,as is shown in FIGS. 7A and 7B.

[0096] With CPAP, a single pressure is generated and deliveredthroughout the sleeping session. PPAP can be advantageously joined withCPAP to lower the pressure provided to the patient during exhalation.The resulting equations for pressure delivered under combined PPAP-CPAPare as follows:

Pinhalation=CPAP

[0097] and

Pexhalation=CPAP+Gain_(Exp)*Flow

[0098] where:

[0099] “Gain_(Exp)” is the constant used during exhalation (negativeflow) to reduce pressure based on the flow rate.

[0100] It should be noted that the CPAP pressure for this embodiment canbe fixed for all time during the therapy session or it can vary over thecourse of the treatment session. For example, it is known to provide apatient with an auto-titrating CPAP pressure, where the CPAP pressure isvaried based on the monitored condition of the patient, the condition ofthe pressure support system, or both. Thus, the present inventioncontemplates combining PPAP during exhalation with an auto-titratingCPAP pressure during inhalation, so that the CPAP pressure is determinedusing any conventional auto-titration technique, such as that taught byU.S. Pat. Nos. 5,203,343; 5,458,137 and 6,087,747 all to Axe et al. orU.S. Pat. No. 5,645,053 to Remmers et al.

[0101] It has further been discovered that patient comfort can beincreased when using the PPAP-CPAP combination. It can happen when usingconventional triggering techniques, that the trigger from inspiration toexpiration is detected before the patient's flow reaches zero. In otherwords, the patient's expiratory effort begins, not at the zero flowpoint, but at a period of time slightly before that point. For example,at point “p” in FIG. 7A the pressure support system may consider thepatient to be in the expiratory phase of the breathing cycle.

[0102] If Pexhalation is calculated at point “p”, where the patient flowis positive, the patient would receive an increase in pressure, whenthey should be getting a decrease in pressure or “pressure relief”. Thiscan be avoided by delaying the calculation of Pexhalation until the flowactually goes below zero. However, the patient may perceive a delay inthe pressure relief Therefore, a further embodiment of the presentinvention contemplates calculating Pexhalation at the trigger frominspiration to expiration as follows:

Pexhalation=CPAP+Gain_(Exp)*(Flow−FlowOffset),

[0103] where “Flow” is the current patient flow and “FlowOffset”corresponds to the last value of patient flow (if positive) duringinspiration. In essence, the FlowOffset value allows Pexhalation to becalculated at point “p” in FIG. 7A, but accounts for the fact that theflow is positive at this point by subtracting out an equal or greateramount of flow using the FlowOffset value. Pexhalation is calculatedusing the FlowOffset only if the Flow is positive. Otherwise, theFlowOffset can be removed or set to zero.

[0104] The value of FlowOffset must be decreased during the expiratoryphase to ensure that the offset reaches zero by the end of expiration,i.e., when the patient's flow increases back to zero, so that thepressure relief returns to zero at that time. The decrease on FlowOffsetcan be done periodically or continuously throughout the expiratoryphase.

[0105]FIGS. 8A and 8B demonstrate that PPAP can also be combined withbi-level PAP therapy in a number of ways to produce effectivetherapeutic pressure waveforms. One application, generally similar tothe aforementioned PPAP-CPAP scenario, is to use PPAP to lower thepressure during exhalation. The resulting equations for the delivery ofcomposite PPAP—bi-level PAP pressure are as follows:

Pinhalation=IPAP

and

Pexhalation=EPAP+Gain_(Exp)*Flow

[0106] where:

[0107] “Gain_(Exp)” is the constant used during exhalation (negativeflow) to reduce pressure based on the flow rate.

[0108] Another approach to merging PPAP with bi-level therapy is shownin FIGS. 9A and 9B where IPAP is applied to the patient for a firstportion of the inspiratory cycle and PPAP is applied for the remainderof the breathing cycle. Gain_(Insp) is automatically calculated for eachbreath based on IPAP and the flow rate as follows:

Pinhalation (t₀ to t₁)=IPAP

and

Pinhalation (t₁ to t₂)=Pbase+AutoGain_(Insp)*Flow

and

Pexhalation=Pbase+Gain_(Exp)*Flow

[0109] where:

[0110] “Flow” is the estimated flow rate;

[0111] “t₀” is the time at the start of breath;

[0112] “t₁” is the time when the estimated flow rate is a predeterminedpercentage of peak inspiratory flow rate;

[0113] “t₂” is the time at the start of exhalation;

[0114] “IPAP” is a continuously applied inspiratory positive airwaypressure;

[0115] “Pinhalation (t₀ to t₁)” is the pressure delivered to the patentfrom to t₀ t₁;

[0116] “Pbase” is a continuous base pressure;

[0117] “AutoGain_(Insp)” equals (IPAP-Pbase)/Flow at t₁;

[0118] “Pinhalation (t₁ to t₂)” is the pressure delivered to the patientfrom t₁ to t₂;

[0119] “Gain_(Exp)” is the constant used during exhalation to reducepressure delivered to the patient; and

[0120] “Pexhalation” is the pressure delivered to the patient duringexhalation.

[0121] It is to be understood that the flow and PPAP pressure outputcurves of FIGS. 3A through 9B represent the apparatus output pressureand flow during the inspiratory and expiratory phases of a singlerespiratory cycle. The PPAP and flow curves can, of course, be expectedto vary somewhat from respiratory cycle to respiratory cycle dependingon the patient's respiratory requirements, particularly under fullyautomated PPAP therapy described hereinafter. Furthermore, somewhatgreater variations will likely occur between the respiratory cyclesassociated with different stages of an extended treatment session,especially during OSAS treatment.

[0122]FIG. 2 represents a further preferred embodiment of a PPAPapparatus pursuant to the present invention, designated herein byreference numeral 10′. Apart from the addition IPAP/EPAP (bi-level PPAP)circuitry 64, PPAP apparatus 10′ is identical in structure and functionto PPAP apparatus 10. According to this embodiment, output 66 from flowtransducer 22 is fed to bi-level PAP circuitry 64. Bi-level PAPcircuitry 64 may assume any conventional form such as, for example, thatdescribed in U.S. Pat. Nos. 5,148,802; 5,433,193; and 5,632,269, thecontents or which are incorporated herein by reference. Output 68 frombi-level PPAP circuitry 64 is transmitted to the PPAP circuitry 28.Output 68 consists of an IPAP signal if the patient is inhaling and anEPAP signal in the event the patient is exhaling. The logic of the PPAPcircuitry 28 utilizes this input according to a preselected one any ofthe aforementioned combinations of PPAP-bi-level therapy to generate adesired pressure command signal 54.

[0123] Pursuant to the present invention, the pressure delivered to thepatient is determined by the base pressure, the flow rate and the gain(and the pressure profile if used). For a given patient condition, thesesettings can be adjusted as necessary to stabilize the airway. In OSAS,a patient's periodic and, to a lesser extent, instantaneous condition isvariable with sleep state and body position. Thus, settings that maywork well in during one portion of a sleeping session may not work aswell at a different time. In other words, settings that support theairway at its most unstable state may cause pressures that are higherthan necessary during more stable times. Likewise, settings that workwell at one point in the session may be insufficient at another time.

[0124] The present invention proposes several methods to minimize theimpact of the patient's changing needs on the optimization of PPAPtherapy. One such method is to automatically adjust the gain, pressureprofile and baseline pressure to meet the patient's demands. Thisadjustment can be based on analysis of patient parameters related toflow, e.g., magnitude, shape, derivative, integral (volume), pressure,snoring, arterial oxygen saturation, exhaled CO₂, airway diameter, orother parameters.

[0125] Using one or more of these parameters the system may adjust theGain_(Insp) to prevent partial airway obstruction (hypopnea). The goalof such systems is to increase Gain_(Insp) responsive to any of thefollowing patient conditions:

[0126] decreased inspiratory flow;

[0127] decreased inspiratory volume;

[0128] increased airway resistance, as determined by flow or pressuresignal analysis;

[0129] airway instability, as indicated by pressure or sound variations;

[0130] drops in arterial oxygen saturation; or

[0131] decreases in airway diameter.

[0132] The apparatus according to the invention may also maintainminimal Gain_(Insp) in the absence of these conditions.

[0133] The present system may also adjust the base pressure (Pbase) toprevent complete collapse of the airway (apnea) or severe collapse(severe hypopnea). Apnea can be detected by analysis of the flow signaland/or by using reflected pressure waves, or a combination of pressureand flow to determine airway patency. Moreover, it may be important todetermine if the apnea is caused by airway collapse or by a lack ofrespiratory drive. If an obstructive event is detected the base pressurecan therefore be increased to open the airway. A further capability ofthe present system is to maintain a minimum Pbase in the absence ofthese conditions.

[0134] The system may also adjust the pressure profile (Pprofile) toprevent apnea or hypopnea at the onset of inspiration. As such, thesystem may increase Pprofile in response to decreased inspiratory flow,decreased respiratory volume, flow waveform shape analysis thatindicates increasing airway resistance, pressure or sound variationsindicative of airway instability, drops in arterial oxygen saturation,decreases in airway diameter or a change in exhaled CO₂. Commensurate,therewith, the present invention also functions to maintain the minimumpressure profile in the absence of these conditions.

[0135]FIG. 10 reveals a presently preferred embodiment of a fullyautomated PPAP apparatus 10″ constructed according to the presentinvention. Generally similar in structure and function to PPAP apparatus10 of FIG. 1, PPAP apparatus 10″ additionally incorporates amicroprocessor or central processing unit (CPU) 70 that preferablyutilizes an output signal 72 from flow transducer 28 as a continuousfeedback signal to enable the CPU to continuously adjust Pbase,Pprofile, Gain_(Insp), and Gain_(Exp) as necessary. The CPU may,however, be configured to effect its continuous system control functionsresponsive to any of the aforementioned patient parameters related orunrelated to respiratory flow.

[0136] Apparatus 10″ also has the capability to detect hypopnea, asevidenced by decreases in peak flow and/or tidal volume for a givenperiod of time, and the occurrence of apneas, as manifested by verylittle flow for a given period of time. To detect hypopnea, for example,the CPU 70 may be programmed to make a comparison between a short termaverage of peak inspiratory flow rate or tidal volume (e.g., a 3 breathaverage) and a long term average of peak flow rate or tidal volume(e.g., greater than 20 breaths). If a decrease of greater than 25% isdetected the system determines a hypopnea to be present. Thisdetermination is desirably made only if the leakage is well estimatedand stable. Thus, large changes in leak or initiation of a leak recoverywill cause data to be ignored.

[0137] The invention further includes a method for determining if theairway is open (central apnea) or obstructed (obstructive apnea) duringan apnea. Once an apnea of significant duration is detected the system,under the direction of CPU 70, automatically increases Gain_(Insp) by 2cm H₂O, waits approximately 1 second and decreases the pressure back tothe original value. If there is a significant change in flow during thispressure change, the system concludes that the airway is open (centralapnea). If there is no significant change in flow the system determinesthat the airway is obstructed (obstructive apnea). The system willcontinue to monitor each apnea for its entire duration at periodicintervals to determine the nature of the apnea.

[0138] In accordance with a preferred embodiment, the PPAP apparatus 10″controls are automatically adjusted as follows. In the event of ahypopnea, Gain_(Insp) is increased by 2 cm/liter/second. In the event ofan obstructive apnea, Pbase is increased by 1 cm H₂O. The device willcontinue to increase Pbase as long as an obstructive apnea ofsignificant duration is detected. The device will not increaseGain_(Insp) again, if necessary, until 5 breaths have passed. If nohypopnea or apneas occur over a period of 30 breaths, Gain_(Insp) isdecreased by 1 cm/liter/second. If no hypopnea or apneas occur over aperiod of 50 breaths, Pbase is decreased by 1 cm H₂O. In addition, theapparatus may control the delivery of O₂ while patient flow is greaterthan zero, if such desired or necessary.

[0139] Although not illustrated, still further embodiments of thepresent invention contemplate the incorporation of fully automated PPAPwith CPAP and/or bi-level PAP therapy. In these cases CPAP or IPAP maybe controlled using the same logic that controls Gain_(Insp) in theabove-described fully automated PPAP system. Likewise, Pbase may becontrolled in a similar manner to that described in connection withfully automated PPAP.

[0140] The fully automated PPAP-CPAP or PPAP-bi-level PAP systems mayalso adjust Pprofile to prevent apnea or hypopnea at the start ofinspiration. Such systems may therefore increase CPAP (or IPAP) orPprofile in the face any of the following patient conditions:

[0141] decreased inspiratory flow;

[0142] decreased inspiratory volume;

[0143] increased airway resistance, as determined by flow or pressuresignal analysis;

[0144] airway instability, as indicated by pressure or sound variations;

[0145] drops in arterial oxygen saturation; and

[0146] decreases in airway diameter.

[0147] It will be understood that CPAP or IPAP would be maintained atminimal levels in the absence of these conditions.

[0148] Using PPAP therapy, therefore, it is additionally possible toemploy PPAP in response to expiratory flow to reduce pressure appliedduring expiration to less than the patient's PEEP level throughout allbut the end of the expiratory phase in a manner similar to thatdescribed for lowering the pressure below Pbase during exhalation in thetreatment of OSAS. This lowering of applied pressure to less than PEEPduring the expiratory phase diminishes breathing work and enhancespatient comfort when compared to the constant expiratory phase pressureapplied during EPAP. Indeed, PPAP can be adapted to any ventilation modethat uses PEEP. Such applications may include pressure support withPEEP, PAV with PEEP or other applications of PEEP in respiratoryassistance therapy.

[0149] As noted above, the present invention contemplates providingdifferent inspiratory gains or expiratory gains depending on thepressure support needs of the patient. See, e.g., FIG. 4B and theassociated text. A further embodiment of the invention contemplatesallowing the patient to select the gain (inspiratory or expiratory) sothat the user can try different gains to determine which gain providesthe best pressure support therapy. It can be appreciated that thisembodiment is best suited for allowing the patient to select theexpiratory gain, such as when using the PPAP-CPAP combination, becauseit is commonly accepted that the inspiratory positive pressure, and,hence, the inspiratory gain, should be set by a physician and should notbe altered by the patient.

[0150] For example, the present invention contemplates having a userselect a first expiratory gain setting and use the pressure supportsystem at that gain. The efficacy, comfort, or both of the pressuresupport system operating at the first gain, can then be monitored usingany conventional technique, such as by having the patient complete aquestionnaire. See, for example, published PCT application publicationno. WO00/18347, which teaches a technique for providing a pressuresupport system and an efficacy questionnaire. In a subsequent therapysession, the user selects a second expiratory gain different than thefirst. The efficacy, comfort, or both of the pressure support systemoperating at the second gain can then be monitored and compared againstthe results garnered using the first gain. This process can be repeatedusing still other gains. In this manner, the optimal gain can bedetermined. The present invention also contemplates allowing the user toadjust the base pressure to determine its optimum level using theabove-described technique.

[0151] The present invention further contemplates that theabove-described patient-feedback determination of the optical gain orbase pressure can be performed automatically. For example, the pressuresupport system can be programmed to automatically provide differentgains or base pressures during different therapy sessions. After eachsession the user can be prompted to take a survey to gauge theeffectiveness, comfort, or both of that therapy session. Aftersufficient data is collected the system can then automatically selectthe gain or base pressure based on the survey results that is optimumfor that patient.

[0152] Furthermore, the administration of oxygen in phase withinspiration may also easily be included with PPAP therapy for thetreatment of COPD patients requiring supplemental oxygen.

[0153] The present invention also contemplates that the pressure of thegas being provided to the patient can be controlled so as to vary overtime. For example, in one embodiment of the present invention, thepressure provided to the patient increases from a first minimum pressureto a desired therapy pressure over a period of time. This ramp increasein pressure provides the patient with time to fall asleep underrelatively low pressure that is increased to the therapy pressure overtime. Thereafter, the pressure increases so that the therapy pressure isbeing applied after the patient is asleep. A reverse process can beperformed in the morning, with the pressure being decreased from thetherapy pressure shortly before the patient intends to wake up. Rampcontrol 120 in FIG. 10 schematically illustrates a manually actuatedcontroller that provides commands to PPAP circuitry 28 to cause thepressure to be provided according to a ramp cycle. Furthermore, the rampcontrol may be adjusted according to the output of CPU 70. Ramp control120 can be used to set the parameters associated with the ramp function,such as the ramp period, ramp start time, ramp stop time, and rampshape.

[0154] Examples of techniques for controlling the pressure levelprovided to the patient via one or more ramp functions, as well as othermethods for controlling the patient pressure, are disclosed in U.S. Pat.Nos. 5,492,114; 5,551,418 and RE 35,295, the contents of each areincorporated herein by reference. Many of the techniques taught by thesepatents can be incorporated into the present apparatus and method toprovide the optimum therapy necessary to treat the patient.

[0155] In a still further embodiment of the present invention, an alarm122 is coupled to PPAP circuitry 28 and/or CPU 70. Alarm 122 can becontrolled so as to be actuated as a result of a variety ofcircumstances. However, in a preferred embodiment of the invention,alarm 122 is actuated responsive to an automatically determined gainfalling outside a predetermined range of values.

[0156] The present invention also contemplates limiting a value for anautomatically determined gain, such as AutoGain_(Insp) discussed above,to prevent the automatically determined gain from exceedingpredetermined limits, for example, from exceeding limits that may resultin an excessively high pressure being provided to the patient. Thelimits on the amount that the gain can themselves be altered so thatthese limits vary over a predetermined period of time. Also, the amountof change that may take place in the automatically determined gain overa predetermined period of time can also be controlled, therebypreventing the automatically determined gain from changing by more thana predetermined amount over the predetermined period of time.

[0157] In using PPAP to treat CHF, the present invention reduces meanpressure and work of exhalation while still providing the same level ofrest to the heart. By applying a positive base pressure substantiallyequivalent to a pressure needed to reduce cardiac preload and afterload(preferably in the range of 5-10 cm H₂O), the present invention helpsthe heart reduce its efforts. With additional positive pressure duringinspiration in proportion to respiratory effort, one can overcome theeffect of negative pressure being produced during inspiration. PPAP isparticularly appropriate in CHF patients in that the typical CHF patienthas normal lung compliance. In these patients, much of the respiratoryloading can be inferred from the flow signal. By reducing the pressurebelow the base pressure during exhalation, one can reduce the work ofexhalation without, reducing the benefit to the heart. The net effectwill be the same benefit to the heart with reduced work of breathing andlower mean pressure.

[0158] Similar to PPAP therapy's use for preventing airway collapse,PPAP therapy for the treatment of CHF delivers only the minimum amountof pressure needed to reduce cardiac preload and afterload. This willresult in supplying a base pressure to the exterior of the heartequivalent to the pressure needed to reduce cardiac preload andafterload in the absence of respiratory loading and a varying pressurewhich is needed to overcome the impact of respiratory loading on cardiacpreload and afterload while minimizing the work of breathing.

[0159] Supplying positive pressure to the exterior of the heart via therespiratory system has two benefits firstly, the positive pressure willreduce the enlarged heart of a CHF patient to a size closer to normal.This return to normal size, allows the muscles of the heart to work moreeffectively. Secondarily, the positive pressure in the chest cavityreduces the amount of pressure the heart must overcome to pump blood tothe rest of the body.

[0160] The heart and chest cavity are at the same pressure. Typicallythis pressure fluctuates about ambient pressure due to the impact ofrespiratory loading. The circulatory system has a working pressure thatvaries as the heart pumps but averages 100 mm HG in normal-tensivepatients. The heart must supply the power to force blood from the chestcavity into the pressurized circulatory system. Increasing the pressurein the chest cavity reduces the amount of pressure the heart must overcome to pump blood. A pressure in the chest cavity of 10 cm H₂O orapproximately 10 mm Hg will reduce the load on the heart by 10 mm Hg/100mm Hg or roughly 10%.

[0161] The impact of respiratory effort on the heart is as follows:during inspiration, the pressure in the chest (and thus surrounding theheart) becomes more negative relative to the rest of the body. Thisincreased negative pressure increases the amount of pressure the heartmust generate to pump blood from the chest cavity to the body. Byproviding pressure in excess of the base pressure during inspiration,PPAP is able to offset this decrease in chest cavity pressure andmaintain a relatively constant pressure in the chest.

[0162] During exhalation the pressure in the chest becomes less negativerelative to the rest of the body. By reducing the pressure duringexhalation, PPAP is able to offset the increase in chest cavity pressureand maintain a relatively constant pressure in the chest.

[0163] By minimizing the decrease in pressure and taking advantage ofthe increased pressure during exhalation, the variable portion of PPAPallows a lower baseline to be set relative to using a constant pressurewith the same benefits to the heart. This lower baseline and reducedpressure during exhalation also reduces the work of breathing andincreases patient comfort.

[0164] It is further desirable to implement a version of PPAP similar tothat discussed above with respect to FIG. 7 on existing CPAP devices.Providing a version of PPAP on existing CPAP devices enhances thepatient comfort and, hence, compliance without the significant financialand other burdens appurtenant to manufacturing and introducing a newCPAP device that includes a PPAP mode. Providing a PPAP therapy on aCPAP device can be accomplished in a variety of ways, such as thatdiscussed above with respect to FIGS. 7A and 7B.

[0165] A version of PPAP can be implemented on a CPAP system in a morecost effective manner if the reactive component used to generate thereduced pressure curve during exhalation in FIG. 7 is replaced with adefined reduced pressure profile. This pressure profile replaces theconstant CPAP pressure otherwise applied by the CPAP device during theexpiratory phase of the patient's breathing cycle. In an preferredembodiment of the present invention, the defined pressure profile has ashape that generally corresponds to a patient's normal flow.

[0166]FIGS. 11A and 11B are flow and pressure diagrams similar to FIGS.7A and 7B illustrating a resultant apparatus pressure output curveaccording to a further embodiment of the present invention that utilizesa simplified pressure profile generating technique. Flow signal 80 inFIG. 11A illustrates the patient's inspiratory phase 82 and expiratoryphase 84. As shown in FIG. 11B, a continuous CPAP pressure 86 isdelivered during the inspiratory phase 82. During the expiratory phase,the pressure support device is controlled to deliver a reduced pressurefollowing a predetermined pressure profile 88. The resulting equationsfor pressure delivered under the combined CPAP and PPAP are as follows:

Pinhalation=CPAP

and

Pexhalation=CPAP−Predetermined Pressure profile.

[0167] The predetermined pressure profile, which is used to reduce theCPAP pressure, has a magnitude M, which is typically selected by arespiratory therapist in a range of 0-4 cmH₂O, and a duration D that,unlike the pressure curve in FIG. 7B, is not directly determined base onthe patient's instantaneous flow or volume. The magnitude M representsthe drop in pressure from the constant CPAP value. The duration D valueis preferably a fraction of an average expiration period of the patient.

[0168] Multiple predefined pressure profiles, having differentmagnitudes, durations or both can be stored in a CPAP/PPAP device andprovided to the patient. FIG. 11B illustrates three predeterminedpressure profiles P1, P2 and P3, having magnitudes M1, M2 and M3 anddurations D1, D2 and D3, respectively. In a preferred embodiment of thepresent invention, the pressure profiles are selected so that thepressure provided to the patient during exhalation roughly correspond tothe contour generated from flow or volume based PPAP, as shown, forexample, in FIGS. 3A and 3B. As shown in FIG. 11B, the pressure dropsoff quickly at the start of expiration then rises slowly toward thebaseline CPAP pressure.

[0169] Because this embodiment for a CPAP/PPAP device does not controlthe flow and/or pressure provided to the patient based on the flow orpressure signal from the patient, as is the case with the PPAP devicesand techniques discussed above, but instead, merely detects the start orexpiration and/or inspiration, the sensor required by this embodimentneed not be as accurate as in the previous embodiments. For example, athermister or thermocouple could replace the costly pneumotach flowmeter to determine the inspiratory and expiratory state. Also, thepressure profile provided to the patient during the expiratory phase canbe generated using motor speed control, thereby avoiding the use of apressure control valve.

[0170] While the above embodiment has been described above with respectto the use of a predetermined pressure profiled used to reduce a CPAPpressure during expiration, the same technique can be applied to abi-level pressure support device to achieve a pressure curve shown, forexample, in FIG. 8B by reducing the bi-level EPAP pressure by thepredetermined pressure profile.

[0171] It has been observed that in some cases where PPAP is implementedand the gain is set above a certain amount, for example beyond the rangeof 2-3 cm/liter/sec, there is a tendency for the pressure generated bythe device to become unstable. More specifically, as shown in FIG. 12,oscillations 90 occur in the pressure waveform 92 applied to the patientduring portions of the patient's expiratory phase 94. These oscillationstypically occur after the initial pressure drop following the onset ofthe expiratory phase and are believed to be generated as a result of theinteraction of the patient's flow and the resulting pressure decrease.

[0172] Despite the chance of such oscillations occurring, it is stillpreferable to provide a relatively large decrease in the pressure beingprovided to the patient at the onset of expiration while maintaining thepressure profile as smooth as possible during the remainder of theexpiratory phase. This is accomplished according to one embodiment ofthe present invention by providing pressure to the patient duringexpiration and after the maximum pressure reduction according to thefollowing equations:

Pexhalation=the greater of: Pbase+Gain_(Exp)*Flow or a Current Pressure,

[0173] where the “Current Pressure” is the pressure being provided tothe patient at that time during the expiratory phase.

[0174]FIG. 13 illustrates a pressure profile 92′ similar to profile 92illustrated in FIG. 12, except that the oscillations occurring duringthe expiratory phase have been removed using the above describedtechnique. For example, at point 96, which is after the maximumreduction in pressure at point 95, the pressure begins to decrease fromthe immediately previous pressure due to a pressure oscillation. Theabove oscillation prevention technique prevents this decrease bysubstituting the current pressure, i.e., the pressure at point 96 at allpoints thereafter where the calculated pressure, i.e.,Pbase+Gain_(Exp)*Flow, is less than the current pressure, therebycreating a plateau section 98 that corresponds to the pressure at point96, i.e., the current pressure, until a point 100 where the calculatedpressure becomes greater than the current pressure. It should be notedthat for purposes of this invention Flow is considered to be negativeduring the expiratory phase.

[0175] By ensuring that the pressure provided to the patient is alwaysthe greater of the current pressure and the calculated pressure, thepressure received by the patient during expiration does not oscillate.If the pressure to be provided to the patient begins to decrease belowthe current pressure, the device will not use the calculated pressure,but will continue to provide the patient with the current pressure,thereby preventing a pressure decrease below the current pressure.

[0176] It can happen, however, that after a first minimum flow isreached, the flow will again drop below that first minimum. For example,FIG. 15 illustrates an example where the patient flow reaches a firstminimum in region 150, increases in region 152 and then falls below thefirst minimum in region 154. If this occurs, the pressure to bedelivered to the patient should not be clamped at the current pressurePcurrent. Instead, the pressure to be delivered should be calculatedusing the PPAP technique, i.e., Pexhalation=Pbase+Gain_(Exp)*Flow. Thus,in region 154, where the patent flow has again fallen to a level belowthe previous minimum level, the pressure delivered is calculated asPexhalation=Pbase+Gain_(Exp)*Flow. Of course, the present inventioncontemplates that Pbase can be a CPAP or IPAP pressure, as noted above.

[0177] In a second embodiment of the present invention, the pressureoscillations are avoided by using an entirely different calculation fordetermining the pressure to be provided to the patient during theexpiratory phase. Instead of basing the calculation of the pressure tobe provided to the patient based on the patient flow multiplied by again, which is selected either manually or automatically, as in theprevious embodiments, the calculation of the pressure to be provided tothe patient is based on the volume of gas still contained in the lungs,referred to as the volume to be exhaled. The volume to be exhaledcorresponds to a difference between a current volume of gas in thepatient and a volume of gas in the patient at rest. The volume of gascurrently in the lungs can be readily estimated from the flow signal.The volume of gas in the patient at rest is determined usingconventional techniques, and can be updated on a periodic base to ensurethe accuracy of the calculation.

[0178] According to this embodiment, the pressure output by the PPAPdevice at least during a portion of the expiratory phase is described bythe following function, which can be encoded into the PPAP circuitry:

Pexhalation=Pbase−(Volume_(to be exhaled)*Gain_(Exp))

[0179] where:

[0180] “Pbase” is the base line pressure (greater than or equal to zeroand conceptually equal to EPAP);

[0181] “Volume_(to be exhaled)” is the difference between the currentvolume of gas in the patient less the volume of gas in the patient atrest; and

[0182] “Gain_(Exp)” is the constant used during expiration (negativeflow) to reduce pressure.

[0183] The pressure output during the inspiratory phase is determinedusing the techniques discussed above. FIG. 14 illustrates a pressurecurve 102 generated using the above equation to determine the pressureto be provided to the patient. It can be appreciated from FIG. 14 thatpressure curve 102 accomplishes the functions of lowering the pressureduring expiration and returning the pressure to the baseline at the endof the expiratory phase, preventing airway collapse.

[0184] Because the volume of gas to be exhaled (Volume_(to be exhaled))is relatively large at the onset of the expiratory phase, the pressuredrop at the beginning of exhalation can be quite large. It is preferableto smooth the large drop at the onset of the expiratory phase byincluding a dampening factor in the calculation of the pressure to beprovided to the patient.

[0185] There are many techniques that can be used to dampen the initialpressure drop at the start of the expiratory phase. However, accordingto a preferred embodiment of the present invention, the pressureprovided to the patient during the expiratory phase is describedaccording to the following equation:

[0186] from the start of the expiratory phase (t₀) to X:${Pexhalation} = {{Pbase} - {\frac{t}{X}\left( {{Volume}_{{to}\quad {be}\quad {exhaled}}*{Gain}_{Exp}} \right)}}$

[0187]  and from X to the end of the expiratory phase (t₁):

Pexhalation=Pbase−(Volume_(to be exhaled)*Gain_(Exp))

[0188]  where:

[0189] “t” is a current time following the start of the expiratoryphase; and

[0190] “X” is a predetermined transition point after the start of theexpiratory phase determined by time, or analysis of the flow signal,such as curvature, percent drop from peak flow rate, integration,derivative, analysis of prior breaths or a combination of flow analysisand time.

[0191] As shown in FIG. 14, the value of X is chosen so that thepressure provided to the patient during the initial period from t₀ to Xis calculated taking into consideration the dampening factor t/X,thereby reducing the pressure drop at the onset of exhalation.Thereafter, the dampening factor is not taken into consideration and theexhalation pressure to be applied to the patient is calculated accordingto the second of the above two equations. Thus, this embodiment of thepresent invention provides a smooth pressure curve throughout theexpiratory phase while ensuring that the initial pressure drop at thestart of the exhalation is within expectable parameters.

[0192] It is to be understood that any other dampening technique forsmoothing the size of the initial pressure drop at the start of theexpiratory phase can be used in this embodiment. Thus, the presentinvention is not limited to the dampening technique discussed above.

[0193] It can be appreciated that, in one embodiment of the presentinvention, the pressure to be delivered to the patient (Pinhalation,Pexhalation) determined using the above-described techniques, issubstantially continuously determined during the patient's breathingcycle. Thus, the system is continuously calculating a new setpointpressure to be delivered to the patient and controlling the pressuresupport system via the pressure controller to reach that setpointpressure. In a further embodiment of the present invention, the setpointpressures (Pinhalation, Pexhalation) determined using theabove-described techniques are filtered using a low pass filter. This isdone to smooth the transition from one calculated setpoint pressure tothe next. In a preferred embodiment of the present invention, the lowpass filter has a 50 msec time constant.

[0194] In a still further embodiment of the present invention, the rateof change for the setpoint pressure delivered to the patient ismonitored and controlled to prevent sharp changes in the deliveredpressure. During PPAP therapy, the limits on rate of change of thesetpoint are limited to keep changes in the pressure to be delivered tothe patient more comfortable for the patient. Rapid changes in thesetpoint pressure may impact the patient's natural waveform. By limitingthis impact, the result is a more comfortable therapy, especially in thePPAP-CPAP combination. In addition, increasing the pressure to bedelivered to the patient too quickly may cause the undesirable effect ofan apparent early transition from expiration to inspiration.

[0195] The present invention contemplates controlling the rise rate,i.e., any increase in pressure, and/or the fall rate, i.e., any decreasein pressure, so that the rise rate and/or the fall rate is held withinset limits. For example, the present invention contemplates thatincreases in the setpoint pressure to be delivered to the patient bemade slower at the end of expiration. Thus, the rise rate is morelimited at the end of the expiratory phase than the rate at which fallrate is limited at the start of the expiratory phase.

[0196] In an exemplary embodiment of the present invention, the rate ofchange is controlled as follows. For positive changes in pressure, ifthe current calculated setpoint pressure is greater than the previouscalculated setpoint pressure plus a rise rate limit, the currentsetpoint pressure is set to the previous setpoint pressure plus the riserate limit, otherwise the current calculated pressure is used as thesetpoint pressure. For negative changes in pressure, if the currentcalculated setpoint pressure is less than the previous setpoint pressureplus a fall rate limit, the current pressure to be delivered to thepatient is set to the previous setpoint pressure plus the fall ratelimit. Otherwise, the calculated setpoint pressure is used. This featureof the present invention can be applied to the calculated Pinhalationsetpoint pressure or the calculated Pexhalation setpoint pressure toensure that pressure changes during either or both phase of thebreathing cycle do not occur too sharply.

[0197] It can be appreciated that the setpoint pressure (Pinhalation orPexhalation) need not be calculated continuously. For example, thesetpoint pressure can be calculated using the above-described PPAPtechniques during different stages of the respiratory cycle. In whichcase, filtering other rate of change control techniques can be used tosmooth the transition from one setpoint pressure to the next.

[0198] Although the invention has been described in detail for thepurpose of illustration based on what is currently considered to be themost practical and preferred embodiments, it is to be understood thatsuch detail is solely for that purpose and that the invention is notlimited to the disclosed embodiments, but on the contrary, is intendedto cover modifications and equivalent arrangements that are within thespirit and scope of the appended claims.

What is claimed is:
 1. An apparatus for delivering pressurized breathinggas to an airway of a patient, the apparatus comprising: a gas flowgenerator; a patient interface adapted to couple the gas flow generatorto an airway of a patient; a sensor adapted to detect a fluidcharacteristic associated with a flow of breathing gas within thepatient interface and to transmit a signal corresponding to thecharacteristic; a pressure controller associated with at least one ofthe gas flow generator and the patient interface to control a pressureof the flow of breathing gas provided by the gas flow generator; controlmeans, receiving the signal from the sensor, for controlling thepressure controller so as to cause the flow of breathing gas to bedelivered to the patient at a minimally sufficient pressure during atleast a portion of a breathing cycle to prevent airway collapse, whereinthe minimally sufficient pressure is a summation of a pressure needed toovercome mechanical airway collapsing forces and a pressure needed toovercome airway collapse due to respiratory effort; and a first selectorunit operatively connected to the control means to selectively establisha first gain, the control means controlling the pressure controller soas to deliver the flow of breathing gas at the minimally sufficientpressure during at least a portion of a breathing cycle based on thefirst gain and the signal from the sensor.
 2. The apparatus of claim 1,further comprising a second selector unit operatively connected to thecontrol means to selectively establish a second gain, the control meansapplying the first gain to a first signal output from the sensor, thefirst signal corresponding to a fluid characteristic indicative ofinspiration, and applying the second gain to a second signal output fromthe sensor, the second signal corresponding to a fluid characteristicindicative of expiration.
 3. The apparatus of claim 1, wherein thecontrol means controls the pressure controller so as to prevent pressureoscillations during expiration.
 4. The apparatus of claim 3, wherein thecontrol means controls the pressure controller to prevent pressureoscillations by causing the pressure controller to provide a pressure tothe patient during expiration after a maximum pressure reduction hasbeen met at a greater of (1) the minimally sufficient pressure, and (2)a current pressure being provided to the patient.
 5. The apparatus ofclaim 1, wherein the control means controls the pressure controller toprovide a continuous positive pressure during an inspiratory phase ofthe patient's breathing cycle and to provide the minimally sufficientpressure based on the first gain and the signal from the sensor duringan expiratory phase of the patient's breathing cycle.
 6. The apparatusof claim 1, wherein the control means controls the pressure controllerto provide a first pressure at a first level during at least a portionof an inspiratory phase of a patient's breathing cycle and to provide asecond pressure during an expiratory phase of said breathing cycle,wherein the second pressure has a second level that is lower than thefirst level and is further reduced by an amount based on the first gainand said signal from said sensor.
 7. The apparatus of claim 1, whereinthe control means also limits a rate of change for the pressure of theflow of breathing gas to be delivered to the patient.
 8. The apparatusof claim 7, wherein the control means set a first rate of change limitfor an increase in the pressure to be delivered to a patient and setssecond rate of change limit for a decrease in the pressure to bedelivered to a patient.
 9. The apparatus of claim 1, further comprisingmeans for smoothing transitions between changes in a pressure of theflow of breathing gas to be delivered to a patient.
 10. A method ofproviding pressured breathing gas to an airway of a patient, the methodcomprising the steps of: supplying a flow of breathing gas to an airwayof a patient from a source of gas via a patient interface; determining afluid characteristic associated with the flow of breathing gas withinthe patient interface and outputting a fluid characteristic signalindicative thereof, establishing a first gain to be applied to the flowrate signal; and controlling the flow of breathing gas to the patientduring at least a portion of a breathing cycle based on the fluidcharacteristic signal and the first gain so as to deliver the flow ofbreathing gas to a patient at a minimally sufficient pressure to preventairway collapse, wherein the minimally sufficient pressure is asummation of a pressure needed to overcome mechanical airway collapsingforces and a pressure needed to overcome airway collapse due torespiratory effort.
 11. The method of claim 10, further comprising astep of establishing a second gain to be applied to the fluidcharacteristic signal, wherein the step of controlling the flow ofbreathing gas to such a patient includes applying the first gain to afirst fluid characteristic signal indicative of inspiration, andapplying the second gain to a second fluid characteristic signalindicative of expiration.
 12. The method of claim 10, wherein thecontrolling step includes controlling said pressure controller so as toprevent pressure oscillations during expiration.
 13. The method of claim12, wherein controlling the pressure controller so as to preventpressure oscillations includes providing a pressure to such a patientduring expiration after a maximum pressure reduction has been met at agreater one of (1) the first minimally sufficient pressure, and (2) acurrent pressure being provided to such a patient.
 14. The method ofclaim 10, wherein the controlling steps includes (1) providing acontinuous positive pressure during an inspiratory phase of a patient'sbreathing cycle and (2) providing the minimally sufficient pressurebased on the first gain and the signal from the sensor during anexpiratory phase of such a patient's breathing cycle.
 15. The method ofclaim 10, wherein the controlling steps includes (1) providing apositive pressure at a first level during an inspiratory phase of apatient's breathing cycle and (2) providing a second pressure during anexpiratory phase of a breathing cycle, wherein the second pressure has asecond level that is lower than the first level and is further reducedby an amount based on the first gain and the signal from the sensor. 16.The method of claim 10, wherein the controlling step includes limiting arate of change for the pressure of the flow of breathing gas to bedelivered to the patient.
 17. The method of claim 16, wherein thecontrolling step includes setting a first rate of change limit for anincrease in the pressure to be delivered to a patient and setting secondrate of change limit for a decrease in the pressure to be delivered to apatient.
 18. The method of claim 10, further comprising smoothingtransitions between changes in a pressure of the flow of breathing gasto be delivered to a patient.
 19. An apparatus for deliveringpressurized breathing gas to an airway of a patient, comprising: a gasflow generator; a patient interface coupled to the gas flow generator tocommunicate a flow of breathing gas from the gas flow generator with anairway of a patient; a sensor associated with the gas flow generator orthe patient interface that detects a fluid characteristic associatedwith the flow of breathing gas within the patient interface and outputsa signal corresponding to the fluid characteristic; a pressurecontroller associated with the gas flow generator or the patientinterface to control a pressure of the flow of breathing gas deliveredto a patient; an input device; and processing means, adapted to receivethe signal output from the sensor, a continuous positive airway pressure(CPAP), and an expiratory gain Gain_(Exp) from the input device, fordetermining a pressure to be delivered to a patient during inhalation(Pinhalation) as: Pinhalation=CPAP,  and for determining a pressure tobe delivered to such a patient during exhalation (Pexhalation) as:Pexhalation=CPAP+Gain_(Exp)*(Flow−FlowOffset),  where “Flow” is thecurrent patient flow measured by the sensor, where “FlowOffset”corresponds to a final value of patient flow during inspirationresponsive to the final value being positive, otherwise FlowOffest isset to zero, and wherein the processing means controls the pressurecontroller so as to deliver a pressure corresponding to Pinhalationduring an inspiratory phase of a breathing cycle and corresponding toPexhalation during an expiatory phase of a breathing cycle.
 20. Theapparatus of claim 19, wherein the patient interface includes a conduit,and wherein the pressure controller is a valve associated with theconduit that exhausts gas from the conduit to control the pressure ofthe flow of breathing gas delivered to the patient.
 21. The apparatus ofclaim 19, wherein the CPAP level is set via the input device.
 22. Theapparatus of claim 19, wherein the CPAP level is determined by theprocessing means based on an output of the sensor.
 23. The apparatus ofclaim 19, wherein the processing means controls said pressure controllerso as to prevent pressure oscillations during expiration.
 24. Theapparatus of claim 23, wherein the processing means controls thepressure controller to prevent oscillations by causing the pressurecontroller to provide a pressure to a patient during expiration after amaximum pressure reduction has been met at a greater of (1) Pexhalationand (2) a current pressure being provided to such a patient.
 25. Theapparatus of claim 19, wherein the processing means also limits a rateof change for the pressure of the flow of breathing gas to be deliveredto the patient.
 26. The apparatus of claim 25, wherein the processingmeans sets a first rate of change limit for an increase in the pressureto be delivered to a patient and sets second rate of change limit for adecrease in the pressure to be delivered to a patient.
 27. The apparatusof claim 19, further comprising means for smoothing transitions betweenchanges in a pressure of the flow of breathing gas to be delivered to apatient.
 28. The apparatus of claim 27, wherein the means for smoothingcomprises a filter for filtering a signal provided by the processingmeans to the pressure controller, wherein the signal corresponds toPexhalation or Pinhalation determined by the processing means.
 29. Amethod of delivering pressurized breathing gas to an airway of apatient, comprising: generating a flow of breathing gas; sensing acurrent flow (Flow) of the breathing gas and outputting a signalcorresponding thereto; selecting an expiratory gain (Gain_(Exp));setting a continuous positive airway pressure (CPAP) level; controllinga pressure of the flow of breathing gas delivered to a patient duringinhalation (Pinhalation) as: Pinhalation=CPAP; and controlling thepressure of the flow of breathing gas to be delivered to such a patientduring exhalation (Pexhalation) as:Pexhalation=CPAP+Gain_(Exp)*(Flow−FlowOffset),  where “FlowOffset”corresponds to a final value of patient flow during inspirationresponsive to the final value being positive, otherwise FlowOffest isset to zero.
 30. The method claim 29, wherein generating the flow ofbreathing gas includes carrying the flow of breathing gas to an airwayof a patient via conduit, and wherein controlling the pressure of theflow of breathing gas includes exhausting gas from the conduit,controlling the.
 31. The method of claim 29, wherein setting the CPAPlevel is accomplished manually via an input device.
 32. The method ofclaim 29, wherein setting the CPAP level is accomplished automaticallybased on an output of a sensor adapted to monitor a characteristic ofsuch a patient or a characteristic associated with the flow of breathinggas.
 33. The method of claim 29, further comprising preventing pressureoscillations during an expiratory phase of a breathing cycle.
 34. Themethod of claim 33, wherein preventing oscillations by causing thepressure controller to provide a pressure to a patient during expirationafter a maximum pressure reduction has been met at a greater of (1)Pexhlation, and (2) a current pressure being provided to such a patient.35. The method of claim 29, further comprising limiting a rate of changefor the pressure of the flow of breathing gas to be delivered to thepatient.
 36. The method of claim 35, wherein limiting the rate of changeincludes providing a first rate of change limit for an increase in thepressure to be delivered to a patient and a second rate of change limitfor a decrease in the pressure to be delivered to a patient.
 37. Themethod of claim 29, further comprising smoothing transitions betweenchanges in a pressure of the flow of breathing gas to be delivered to apatient.
 38. The method of claim 37, wherein smoothing transitionsincludes filtering the Pexhalation determinations made during the stepof controlling a pressure to be delivered to such a patient duringexhalation.
 39. The method of claim 29, wherein controlling a pressureof the flow of breathing gas to be delivered to a patient duringexhalation is done substantially continuously.